Medical diagnostic imaging has evolved as an important non-invasive tool for the evaluation of pathological and physiological processes. Presently, nuclear magnetic resonance imaging (MRI) is one of the most widely used imaging modalities, demonstrating many advantages over other techniques like computed tomography. The innocuousness of the applied magnetic field typically used for clinical diagnosis results in its safe use. Proton MRI is based on the principle that the concentration and relaxation characteristics of protons in tissues and organs, when submitted to a static magnetic field, can influence the intensity of a magnetic resonance image. As a result, this technique brings a high contrast resolution, which allows for morphological imaging thanks to contrast tissue differentiation due to differences in proton density of the different tissues (depending on fat or water content). A functional imaging of certain organs can also be performed, i.e., blood oxygenation, or cerebral area activation under sensorial stimuli. MRI also demonstrates a high spatial resolution, as cellular imaging is envisaged with a strong magnetic field and no depth limitations. Although MRI can be performed without the administration of contrast agents, the ability of many contrast enhancement agents to enhance the visualization of internal tissues and organs has resulted in its widespread use. Contrast enhancement agents that are useful for proton MRI are responsible for a change in the relaxation characteristics of protons, which can result in image enhancement and improved soft-tissue differentiation.
The use, as a drug delivery vehicle, of thermosensitive liposomes losing their structural integrity within a given temperature range is a promising approach to targeting a tumor or other tissue, but so far no efficient liposomal MRI contrast agents have been proposed.
US 2004/0101969 describes a method of monitoring the localisation and distribution of a compound of interest released from an envirosensitive liposome, using a molecular compound, MnSO4, as a contrast agent.
WO 2008/035985 describes a trackable particulate material for drug delivery comprising a matrix or membrane material, a drug, an internal T1 magnetic resonance metal chelate contrast agent and an external T1 magnetic resonance metal chelate contrast agent, wherein the internal T1 agent is shielded from bulk water and the external T1 agent is exposed to bulk water.
EP 2067485 describes thermosensitive liposomes for drug delivery which comprise a paramagnetic metal compound. The paramagnetic metal compound may be a metallic nanoparticle. Preferably, the paramagnetic metal compound comprises a chelating structure allowing the metal to interact with water or with another suitable source of protons as a chemical exchange-dependent saturation transfer (CEST) contrast agent.
These documents all refer to contrast agents which are T1 magnetic resonance paramagnetic molecular compounds and offer only a limited sensitivity when measured in vivo. As a result, relatively large, and possibly toxic, doses of such contrast agents are to be administered to a given subject. This and other problems are addressed by the compositions and methods herein disclosed.
WO 2008/033031 describes a trackable MRI drug delivery particle comprising two distinct chemical contrast agents, one of them (T1 agent) exhibiting a T1 signal used to monitor drug release, and the other one (T2* agent) exhibiting a T2* signal. This document does not suggest using the T2* agent to monitor drug release.
US2009/004258 describes thermosensitive liposomes encapsulating iron oxide nanoparticles and carbofluorosceine (CF), CF being used as a drug model. Nanoparticles are capable of generating heat when activated by an alternative magnetic field, thereby allowing permeabilization of the membrane towards CF. In US2009/004258, monitoring of the CF's release is performed by fluorescence detection.
The inventors herein provide an advantageous method offering superior sensitivity, relative to the currently available methods. This method in particular uses low doses of non-toxic charged superparamagnetic nanoparticles and allows the monitoring of liposome delivery to a target site and also, surprisingly, the efficient monitoring of the product of interest's release from said liposomes.